Effective coupling coefficients for strained single crystal epitaxial film bulk acoustic resonators

ABSTRACT

In an array of single crystal acoustic resonators, the effective coupling coefficient of first and second strained single crystal filters are individually tailored in order to achieve desired frequency responses. In a duplexer embodiment, the effective coupling coefficient of a transmit band-pass filter is lower than the effective coupling coefficient of a receive band-pass filter of the same duplexer. The coefficients can be tailored by varying the ratio of the thickness of a piezoelectric layer to the total thickness of electrode layers or by forming a capacitor in parallel with an acoustic resonator within the filter for which the effective coupling coefficient is to be degraded. Further, a strained piezoelectric layer can be formed overlying a nucleation layer characterized by initial surface etching and piezoelectric layer deposition parameters being configured to modulate a strain condition in the strained piezoelectric layer to adjust piezoelectric properties for improved performance in specific applications.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to and is a continuation-in-partapplication of U.S. patent application Ser. No. 15/996,358, titled“IMPROVED EFFECTIVE COUPLING COEFFICIENTS FOR STRAINED SINGLE CRYSTALEPITAXIAL FILM BULK ACOUSTIC RESONATORS,” filed Jun. 1, 2018, U.S. Pat.No. 10,727,811 issued Jul. 28, 2020.

BACKGROUND OF THE INVENTION

The present invention relates generally to acoustic resonators and moreparticularly to controlling the effective coupling coefficient of asingle crystal epitaxial film bulk acoustic resonator.

In many different communications applications, a common signal path iscoupled to both an input of a receiver and an output of a transmitter.For example, in a cellular or cordless telephone, an antenna may becoupled to the receiver and the transmitter. In such an arrangement, aduplexer is often used to couple the common signal path to the input andthe output. The function of the duplexer is to provide the necessarycoupling to and from the common signal path, while preventing thesignals generated by the transmitter from being coupled to the input ofthe receiver.

One type of duplexer is referred to as the half duplexer. A halfduplexer uses a switch to connect the common signal path to the receiveror the transmitter on a time division basis. The half duplexer has thedesired coupling and attenuation properties, but is unacceptable in manytelephony applications, since it does not allow parties of a call tospeak and be heard simultaneously.

A type of duplexer that is more acceptable for telephony applications isthe full duplexer. A full duplexer operates only if the transmit signalhas a frequency that is different than the frequency of the receivesignal. The full duplexer incorporates band-pass filters that isolatethe transmit signal from the receive signal according to thefrequencies. FIG. 1 illustrates a conventional circuit used in cellulartelephones, personal communication system (PCS) devices and othertransmit/receive devices. A power amplifier 10 of a transmitter isconnected to a transmit port 12 of a full duplexer 14. The duplexer alsoincludes a receive port 16 that is connected to a low noise amplifier(LNA) 18 of a receiver. In addition to the transmit port and the receiveport, the duplexer 14 includes an antenna port 20, which is connected toan antenna 22.

The duplexer 14 is a three-port device having the transmit port 12, thereceive port 16 and the antenna port 20. Internally, the duplexerincludes a transmit band-pass filter 24, a receiver band-pass filter 26and a phase shifter 28. The passbands of the two filters 24 and 26 arerespectively centered on the frequency range of the transmit signal thatis input via the power amplifier 10 and the receive signal to which thereceiver is tuned.

The requirements for the band-pass filters 24 and 26 of the duplexer 14are stringent. The band-pass filters must isolate low intensity receivesignals generated at the antenna 22 and directed to the input of the lownoise amplifier 18 from the strong transmit signals generated by thepower amplifier 10. In a typical embodiment, the sensitivity of the lownoise amplifier may be in the order of −100 dBm, while the poweramplifier may provide transmit signals having an intensity ofapproximately 28 dBm. It is expected that the duplexer 14 must attenuatethe transmit signal by approximately 50 dB between the antenna port 20and the receive port 16 to prevent any residual transmit signal mixedwith the receive signal at the receive port from overloading the lownoise amplifier.

One type of PCS that is used in a mobile telephone employs code divisionmultiple access (CDMA). The CDMA PCS wireless bands are centered atapproximately 1920 MHz and have an especially stringent regulatoryrequirement for duplexer performance. Some concerns will be identifiedwith reference to FIG. 2. A passband 30 is defined by several poles andseveral zeros. The poles and zeros are equidistantly spaced from acenter frequency 32. For the transmitter passband 30, thetransmitter-to-antenna insertion loss 34 is preferably better than −3 dBover most of the band. The isolation from the transmitter to receiverports exceeds 50 dB across most of the transmitter band and 46 dB in thereceiver band. The crossover between the transmitter and receiver bandsoccurs around 1920 MHz. The transmitter and receiver bands areapproximately 3.0 percent of the carrier frequency, so that extremelysharp filter roll-off 36 and 38 is required. As will be explained morefully below, the lower-frequency poles and zeroes and the roll-off 36are determined by the characteristics of shunt resonators, while thehigher-frequency poles and zeroes and the roll-off 38 are determined bythe characteristics of series resonators.

Another challenge for the duplexer is achieving power handlingrequirements. The power amplifier 10 of FIG. 1 in the transmitter candeliver 1 Watt of power to the transmit port 12 of the duplexer 14. Theband-pass filter 24 must be capable of handling such power without beingdestroyed and without its performance being degraded.

The duplexer 14 will be described in greater detail with reference toFIG. 3. The duplexer includes a transmit film bulk acoustic resonator(FBAR) array 40 and a receive FBAR array 42. The transmit FBAR array isa two-stage ladder circuit having two series FBARs 44 and 46 and twoshunt FBARs 50 and 52. The series FBARs are connected in series betweenthe transmit port 12 and the antenna port 20, while the shunt FBARs areconnected between electrical ground and nodes between the series FBARs.Each full stage of an FBAR array is composed of one series FBAR and oneshunt FBAR. In order to handle the high power generated by the poweramplifier at the filter input of the transmit filter, power bars areused for each of the series elements 44 and 46.

The receive FBAR array is a 3½-stage ladder circuit. A half stage islimited to either one series FBAR or one shunt FBAR. In the exemplaryarray 42, the half stage is a shunt FBAR 60. The FBAR array includesthree series FBARs 54, 56 and 58 and four shunt FBARs 60, 62, 64 and 66.The series FBARs are connected in series between the ninety degree phaseshifter 28 and the receive port 16. The shunt FBARs are connectedbetween electrical ground and nodes between the series FBARs.

Circuits suitable for use as the ninety degree phase shifter 28 areknown in the art. As examples, the phase shifter may be composed ofinductors and capacitors or may be a .lambda./4 transmission line.

Within the transmit FBAR array 40, each series FBAR 44 and 46 may havethe same resonant frequency (f.sub.r.sup.Tx), which may be centered at1880 MHz. Similarly, the shunt FBARs 50 and 52 may have the sameresonant frequency, but the resonant frequency of the series FBARs isapproximately 1.0 percent to 3.0 percent (typically 1.6 percent) greaterthan that of the shunt FBARs. As a result, the poles that were describedwith reference to FIG. 2 are provided.

The receive FBAR array 42 of the receive band-pass filter 26 may also becomposed of series FBARs 54, 56 and 58 having the same f_(r) ^(Rx) andshunt FBARs 60, 62, 64 and 66 having the same resonant frequency that is3.0 percent different than the resonant frequency f_(r) ^(Rx) of theseries FBARs. Here, f_(r) ^(Rz) is centered at 1960 MHz.

Other considerations that affect the shape of the response shown in FIG.2 are the figure of merit, which is referred to as Q, and the effectivecoupling coefficient, which is also referred to as kt². The effectivecoupling coefficient may be considered as being the ratio of electricalenergy to acoustic energy in the operation of a particular FBAR. It hasbeen the goal to maximize both Q and the effective coupling coefficient.As a result of the fabrication process, the effective couplingcoefficient can be as high as 8.0 percent. It has been experimentallydetermined that Q is dependent upon kt² and, in some cases, that it isbetter to decrease kt² in order to significantly increase Q. The Qdetermines the roll-off of the response.

In addition the considerations discussed above, acoustic resonators

What is needed is a fabrication method and a resulting duplexer whichprovide a very steep roll-off in the operation of an array of acousticresonators further improved with single crystal piezoelectric materials.

BRIEF SUMMARY OF THE INVENTION

The present invention relates generally to acoustic resonators and moreparticularly to controlling the effective coupling coefficient of asingle crystal epitaxial acoustic resonator.

The performance of arrays of acoustic resonators is enhanced by usingsingle crystal piezoelectric materials and tailoring the effectivecoupling coefficients of the individual acoustic resonators on the basisof the functions of the resonators. In a duplexer embodiment, theeffective coupling coefficients of FBARs in a transmit band-pass filterare fabricated to have a lower effective coupling coefficient than theFBARs of the receive band-pass filter of the same duplexer.

In one embodiment, the difference in the effective coupling coefficientsis achieved by varying the thicknesses of the electrode layers. For agiven frequency, the effective coupling coefficient of an acousticresonator is modified by varying the ratio of the thickness of thepiezoelectric layer to the total thickness of the electrode layers.Typically, a goal in the fabrication of FBARs is to minimize thethickness of the electrode layers, thereby providing an “intrinsic”effective coupling coefficient. For example, this intrinsic coefficientmay be in the range of 7.0 percent to 8.0 percent. However, the couplingcoefficient of an FBAR filter having a given resonant frequency can beadjusted downwardly by decreasing the ratio of the thickness of thepiezoelectric layer to the total thickness of the electrode layers,since the resonant frequency is dependent upon the “weighted thickness”(i.e., the physical thickness weighted on the basis of the selection ofelectrode and piezoelectric materials) of the electrode-piezoelectricstack. As one example of a transmit filter, the thickness of molybdenum(Mo) electrodes can be increased and the thickness of aluminum nitride(AIN) can be reduced in order to achieve a degraded effective couplingcoefficient in the range of 2.5 percent to 4.0 percent, whilemaintaining a targeted resonant frequency. Similarly, a receive filtercan be fabricated to have an effective coupling coefficient in the rangeof 4.0 percent to 6.0 percent by selecting the appropriate thicknessesfor the layers that form the FBARs of the receive filter.

The method of fabricating an array of acoustic resonators in accordancewith this embodiment includes a step of selecting a first targetfrequency range and a first target effective coupling coefficient foroperation of an FBAR transmit (Tx) filter, and includes selecting asecond target frequency and a second target coupling coefficient foroperation of an FBAR receiver (Rx) filter. The thicknesses and materialsof the piezoelectric and electrode layers for forming the two FBARfilters are determined on the basis of achieving the target resonantfrequencies and the target effective coupling coefficients. Thedeterminations include selecting an increased electrode layer thicknessfor at least one electrode of the Tx FBARs, so that the Tx FBAR filterwill have the degraded coefficient. The two filters are then formedaccording to the selected thicknesses and materials.

In the fabrication of the two filters, the electrode material may be Moand the piezoelectric material may be AIN. Using these materials, theelectrode layers of the FBAR Tx filter having the degraded couplingcoefficient will have a thickness that can be in the range of 1.2 to 2.8times the thickness of the electrode layers of the Rx filter with thehigher coefficient. For example, in a communications device that iscompatible with the CDMA PCS standard, the Rx filter may have electrodelayer thicknesses of 2200 Å and a piezoelectric thickness of 2.2 micronsin order to achieve a coupling coefficient in the range of 5.6 percentto 5.8 percent, while the Tx filter may have electrode layer thicknessesof 4500 Å and a piezoelectric thickness of roughly 8000 Å in order toachieve a coupling coefficient in the range of 3.1 percent to 3.2percent. The Q (and therefore the steepness of the roll-off) is almosttwo times higher for the Tx filter than for the Rx filter.

In one application, a desired filter arrangement of FBARs is designed toinclude at least one “power bar” in order to increase the power handlingcapacity along a path of the filter arrangement. A “power bar” isdefined herein as a pair of large area FBARs which are connected inseries in place of a single target FBAR. Each large area FBAR occupiesan area that is twice the area of the target FBAR. The parallel-seriescombination defined by the power bar (in the series connection ofconventional electrical equivalent circuits) allows the impedance of thepower bar to remain at the target impedance of the target FBAR, butreduces the power density by a factor of four.

In a second embodiment of the invention, the degraded effective couplingcoefficient is achieved by forming a capacitor in parallel with at leastsome of the resonators of the Tx filter. Preferably, the capacitor isformed using materials that are deposited in steps for fabricating thearray of acoustic resonators. For example, the electrodes and thepiezoelectric layer that are deposited to fabricate the FBARs may beutilized in the formation of a capacitor that is placed in parallel withat least one FBAR of the Tx filter to degrade the effective couplingcoefficient. However, the concern in using these layers is that aresonator will be fabricated, rather than a capacitor. One method forensuring that the additional component functions as a capacitor is tofabricate the electrode-piezoelectric stack of the component directly onthe substrate, rather than suspending the stack. In this manner, thesubstrate provides the means for mass loading the capacitor, therebypulling the frequency off center.

A second method is to use the gold layer, which is conventionally usedto provide contact pads, as the means to pull the resonator componentoff frequency. This second method is preferred, since the first methodmay form a high loss capacitor, while the second method is the one thatwill form a high Q component. By utilizing the gold layer and bysuspending the capacitor component as a free-standing membrane in thesame manner as the FBARs, the capacitor functions as a high Q resonator,but at a much lower frequency than the first and second FBARs. Anadvantage is that the frequency of the capacitor can be “tuned” to notonly be displaced from the frequency of interest, but to form aparasitic resonance at frequencies where the duplexer does not performwell. As one example, the capacitor may resonate at 1510 MHz, which is afrequency at which existing duplexers do not perform well in therejection of energy. Tuning the capacitor to 1510 MHz allows a designerto incorporate specific shunt and series type resonators that reduceleakage of the 1510 MHz signal. This is achieved without any additionalprocess steps to the FBAR fabrication. The tuning of the capacitor canbe provided merely by properly selecting the thickness of the gold andother layers in the electrode-piezoelectric stack of the capacitor.

An advantage of the methods described above is that the performance ofan array of acoustic resonators is enhanced without significantlyaffecting the fabrication process. By tailoring the effective couplingcoefficients of individual resonators within a full duplexer, roll-offat the opposite edges of the passband can be tailored. Further, thepresent method produces a reliable single crystal based acoustic filteror resonator using multiple ways of three-dimensional stacking through awafer process.

A greater understanding of the nature and advantages of the inventionmay be realized by reference to the latter portions of the specificationand attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the present invention, reference ismade to the accompanying drawings. Understanding that these drawings arenot to be considered limitations in the scope of the invention, thepresently described embodiments and the presently understood best modeof the invention are described with additional detail through use of theaccompanying drawings in which:

FIG. 1 a block diagram of a front-end circuit of a conventional cellulartelephone or similar device.

FIG. 2 a graph showing the characteristics of a band-pass filter of thetype used in FIG. 1.

FIG. 3 is a schematic block diagram of a conventional full duplexer.

FIG. 4 is a schematic block diagram of an example of a pair of passbandfilters in which the filters have significantly different effectivecoupling coefficients in accordance with one embodiment of theinvention.

FIG. 5 is a cross sectional view of the selected FBARs of FIG. 4.

FIG. 6 is a process flow of steps for carrying out the inventiondescribed with reference to FIGS. 4 and 5.

FIG. 7 is a schematic diagram of the conventional electrical equivalentcircuit of the FBAR formed in accordance with the first embodiment ofthe invention.

FIG. 8 is a schematic diagram of the electrical equivalent circuit of anFBAR formed in accordance with a second embodiment of the invention.

FIG. 9 is a side sectional view of a resonator-capacitor pair, with thecapacitor being formed directly on the substrate in accordance with thefirst approach to the second embodiment represented in FIG. 8.

FIG. 10 is a side sectional view of a resonator-capacitor pair, with thecapacitor being mass loaded by addition of a top metal layer inaccordance with the second approach to achieving the second embodimentrepresented in FIG. 8.

FIG. 11 is a flow diagram illustrating a method for manufacturing anacoustic resonator device according to an example of the presentinvention.

FIG. 12 is a simplified graph illustrating the results of forming apiezoelectric layer for an acoustic resonator device according to anexample of the present invention.

FIGS. 13A-C are simplified diagrams illustrating methods for forming apiezoelectric layer for an acoustic resonator device according tovarious examples of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to acoustic resonators and moreparticularly to controlling the effective coupling coefficient of asingle crystal epitaxial acoustic resonator.

With reference to FIG. 4, a transmit band-pass filter 68 is shownschematically as including four series FBARs 70, 72, 74 and 76 connectedin electrical series from a transmit port 78 to an antenna port 80. Thefilter also includes two shunt FBARs 82 and 84. The first shunt FBAR 82is connected between the two pairs of series FBARs, while the secondshunt FBAR 84 is connected between the antenna port and the series FBAR76. FIG. 4 also shows one stage of a receive filter 73. This stageincludes a series FBAR 75 and a shunt FBAR 77. The number of stages in atransmit (Tx) filter or a receive (Rx) filter is not critical to theinvention that will be described more fully below. The significance ofthe filters is that at least one FBAR will have an effective couplingcoefficient that is intentionally degraded relative to at least oneother FBAR. In the preferred embodiment, the FBARs of the Tx filter haveeffective coupling coefficients that are intentionally degraded, whilethe FBARs of the Rx filter have significantly higher coefficients.

The FBAR pair 70 and 72 and the FBAR pair 74 and 76 are “power bars.”The series connection of the two FBARs in each pair increases the powerdensity by a factor of four relative to a single target FBAR. Regardingthe first pair of FBARs 70 and 72, each FBAR is fabricated to occupy anarea that is twice the area of the target FBAR. When the two FBARs areconnected in series, the series-parallel arrangement of resistances andcapacitances in the resulting series combination of the conventionalelectrical equivalent circuits (which will be described below withreference to FIG. 7) will achieve the impedance of the target FBAR, butwith the increased power density. The resonant frequencies of the FBARsin a power bar should be the same as the resonant frequency of thetarget FBAR which is “replaced” by the power bar.

The phase shifter 28 of FIG. 4 is not critical to the invention and isconsistent with the description of the phase shifter 28 of FIG. 1. Astwo examples, the phase shifter may be comprised of inductors andcapacitors or may be a one-quarter wavelength transmission line. Each ofthe shunt FBARs 82 and 84 is coupled to ground through an externalinductor 86 and 88. The inductors may be used to position theattenuation poles of the shunt bars, so that the passband responseexhibits the desired characteristics, such as steep roll-off at theoutside edges of the response.

Each of the FBARs 70, 72, 74, 75, 76 and 77 includes outside electrodelayers and an interior piezoelectric layer. For example, the series FBAR74 of the Tx filter 68 includes electrodes 90 and 92 that sandwich apiezoelectric layer 94. Similarly, the series FBAR 75 of the Rx filter73 includes electrodes 96 and 98 and a center piezoelectric layer 100.The ratio of the thickness of the piezoelectric layer 94 to the totalthickness of the electrode layers that sandwich the piezoelectric layeris represented as being much less for the series FBARs of the Tx filterthan for the series FBAR of the Rx filter. As will be explained morefully below, the increased electrode layer thickness degrades thecoupling coefficient of the Tx filter 68 relative to the Rx filter 73.As a result, the Q of the Tx filter is greater than the Q of the Rxfilter and the steepness at the edges of the passband response of the Txfilter is greater than that of the Rx filter.

FIG. 5 illustrates a side sectional view of the series FBAR 74 of the Txfilter and the series FBAR 75 of the Rx filter. Again, the electrodelayers 90 and 92 of the FBAR 74 are shown as being significantly thickerthan the electrode layers 96 and 98 of the FBAR 75. Both of the filters68 and 73 are formed on a single substrate 102, such as a siliconsubstrate. However, the filters may be formed on separate substrates ormay be formed on a material other than silicon. An FBAR is formed wherethe piezoelectric material 94 and 100 is sandwiched between twoelectrodes 90, 92, 96 and 98. Preferably, wells 104 and 106 are etchedinto the substrate below the FBARs. As a result, eachelectrode-piezoelectric stack that forms an FBAR is a membrane suspendedover a well, so as to provide resonator-to-air interfaces at both sides.Alternatively, solidly mounted resonators (SMRs) may be used withoutdiverging from the invention. SMRs typically include acoustic Braggreflectors at their bottom surfaces in order to provide a large acousticimpedance. A Bragg reflector is made of layers of alternating high andlow acoustic impedance materials, with each layer having a thickness ofapproximately one-quarter wavelength of the resonant frequency of theFBAR. In some applications, a number of FBARs share a single well.

The characteristics of the individual series FBARs 74 and 75 of FIGS. 4and 5 depend upon the layer thicknesses and the materials of theelectrode-piezoelectric stacks. The preferred material for forming thepiezoelectric layers 94 and 100 is AIN, but other materials may beutilized (e.g., zinc oxide). An acceptable electrode material is Mo, butother metals may be substituted (e.g., aluminum, tungsten, gold ortitanium). For given electrode and piezoelectric materials,characteristics of an FBAR are dependent upon geometrical factors suchas the thickness of the piezoelectric layer, the thicknesses of theelectrodes and the area of overlap between the electrodes. For example,the resonant frequency is dependent upon the “weighted thickness” of theelectrode-piezoelectric stack. The weighted thickness is the physicalthickness with an adjustment that is based upon the selection of theelectrode and piezoelectric materials. The adjustment is necessary,since the velocity of sound is different in different materials.Changing the weighted thickness of one or both of the electrodes changesthe weighted thickness of the electrode-piezoelectric stack, therebyadjusting the resonant frequency of the stack.

The layer thicknesses of the electrode-piezoelectric stacks also affectthe effective coupling coefficients (kt.sup.2) of the Tx and Rx filters68 and 73. In accordance with the invention, the effective couplingcoefficients of the filters are tailored on the basis of the functionsof the filters. By providing the Tx filter 68 with a lower effectivecoupling coefficient than the Rx filter 73, a CDMA-compatible duplexerexhibits desirable characteristics. As noted with reference to FIGS.1-3, there is a crossover between the transmitter passband and thereceiver passband. The series FBARs 70, 72, 74 and 76 of the Tx filtersignificantly influence the characteristics of the transmitter passbandat the crossover. Intentionally degrading the effective couplingcoefficient while maintaining the specification-required resonantfrequency enhances the duplexer performance. As previously noted, thereduction in the kt.sup.2 of a Tx filter increases its Q, so that asteeper roll-of is achieved.

In FIG. 5, the cross sectional view through the Tx FBAR 74 and the RxFBAR 75 shows the difference in the ratios of the thickness of thepiezoelectric layer and the total thicknesses of the electrode layers.For the Tx FBAR 74, the ratio is significantly less than that of the RxFBAR 75. Therefore, the effective coupling coefficient of the Tx filterwill be significantly less than the coupling coefficient for the Rxfilter. Typically a goal in the fabrication of FBARs is to minimize thethickness of the electrode layers. This provides an intrinsic effectivecoupling coefficient in the range of 7.0 percent to 8.0 percent. In FIG.5, the electrode layers 96 and 98 that are used to define the Rx FBAR 75may be formed of Mo having a thickness of 2200 Å. The portion of thepiezoelectric layer 100 that forms the Rx FBAR may be AIN having athickness of approximately 2.2 microns. This provides thespecification-required resonant frequency for the CDMA-compatibletransmit filter and provides an effective coupling coefficient in therange of 5.6 percent to 5.8 percent.

The Tx FBAR 74 is formed such that the Tx filter 68 will have theintentionally degraded effective coupling coefficient. The Mo top andbottom electrodes 90 and 92 may have a thickness of approximately 4500Å, while the thickness of the relevant portion of the piezoelectriclayer 94 may be approximately 8000 Å. This provides thespecification-required resonant frequency and provides a degradedeffective coupling coefficient in the range of 3.1 percent to 3.2percent.

For Tx and Rx filters 68 and 73 that are formed on the basis of theidentified layer thicknesses, the Tx filter may have a Q that isapproximately twice that of the Rx filter. Consequently, the steepnessat the edges of the Tx filter response will be significantly greater.

Referring now to FIG. 6, the process flow of steps for fabricatingcoefficient-differentiated FBAR filters in accordance with the inventionwill be described. In step 108, a target frequency response and a targeteffective coupling coefficient are selected for a first FBAR filter. Inthe formation of the transmit filter 68 of FIG. 4, the first FBAR filteris the Tx filter 68. The target frequency response will depend upon thedesired application. For example, in a duplexer that is compatible withCDMA requirements, the target frequency response is likely to becentered at 1880 MHz (i.e., f_(r) ^(Tx)=1880 MHz).

In step 110, a target frequency response and a target effective couplingefficient are selected for a second FBAR filter. Again referring to FIG.4, the second FBAR filter is the Rx FBAR filter 73, so that f_(r)^(Rx)=960 MHz. In the preferred embodiment, the first target couplingcoefficient is selected to be in the range of 2.5 percent to 4.0percent, while the second target coupling coefficient is selected to bein the range of 4.0 percent to 6.0 percent.

The layer thicknesses and materials for fabricating the transmit andreceive FBAR filters are determined at step 112. This step includesselecting an increased electrode layer thickness for at least oneelectrode layer of the Tx FBAR filter 68, thereby ensuring that theeffective coupling coefficient of the Tx FBAR filter is degradedrelative to the coupling coefficient of the Rx FBAR filter 73. Thisnecessitates identifying ratios of the piezoelectric layer thickness tothe total thickness of the electrode layers for each of the Tx and RxFBAR filters. The ratio for the Tx FBAR filter will be less than theidentified ratio for the Rx FBAR filter, since the target effectivecoupling coefficient is reduced for the Tx FBAR filter. For purposes ofease of fabrication, the materials for forming the Tx and Rx FBARfilters are preferably the same. However, this is not critical, sincethe difference in the coefficients may be partially achieved byselecting different materials for the two filters.

In step 114, the FBARs are fabricated. The filters 68 and 73 may beformed on the same substrate 102, such as shown in FIG. 5. However,there are process advantages to forming the filters on separatesubstrates and subsequently interconnecting the filters. It is difficultto vary the thicknesses of the electrode layers and the piezoelectriclayers on a single substrate. Forming the FBARs on separate substrateseliminates the difficulties. If the FBARs are to be formed on the samesubstrate, portions of increased layer thickness may be achieved byproviding multiple deposition steps. For example, in the formation ofthe lower electrode layer 92 of FIG. 5, the Mo deposition may betemporarily terminated after the layer has reached its desired thicknessfor the electrode layer 98. A masking layer may then be deposited in thearea of the FBAR 75, so that the re-initiating of the deposition of Mowill occur only in the area of the series FBAR 74. A similar multi-stepdeposition process may be provided for the piezoelectric layer 100. Thetwo top electrodes 90 and 96 may be similarly formed.

While the first embodiment of the invention has been described as beingused in FBARs having a single piezoelectric layer, the invention may beextended to stacked FBARs without diverging from the level of skill inthe art. That is, arrays of FBARs having stacked piezoelectric layersthat are separated by electrode layers may be fabricated to havetailored effective coupling coefficients, so as to achieve desiredfilter characteristics.

The process of modifying the ratio of the thickness of the piezoelectriclayer to the total thickness of the electrode layers is one means fortailoring the effective coupling coefficients of different FBARs in anFBAR array. A second means of tailoring the effective couplingcoefficient is to form capacitors in parallel with selected FBARs. Aswill be explained more fully below, the parallel connection of acapacitor will degrade the effective coupling coefficient. This use of acapacitor to degrade the effective coupling coefficient may be used inapplications other than the design and fabrication of Tx and Rx filters.

FIG. 7 is an electrical equivalent circuit for an FBAR. The circuit isknown in the art as the modified Butterworth-Van Dyke circuit. The mainreactive component is the shunt capacitance (C_(P)) 116, which is thecapacitance defined by the structure of the electrodes and thepiezoelectric layer. The piezoelectric layer functions as the dielectricfor the shunt capacitance 116. The plate resistance (R_(P)) 118represents the series resistance of the shunt capacitance 116, while theresistance (R_(S)) 120 represents the series electrical resistance ofthe connections between the contacts 122 and 124 of theelectrode-piezoelectric stack. Conventionally, the contacts 122 and 124are formed of gold.

The series connections of the inductance (L_(M)) 126, capacitance(C_(M)) 128 and resistance (R_(M)) 130 are the motional representationsof the resonance due to the piezoelectric properties of the FBAR. In theoperation of an FBAR filter having FBARs that are fabricated using thesteps described with reference to FIGS. 5 and 6, the effective couplingcoefficient is directly related to the ratio of the motional capacitance128 to the plate capacitance 116. However, by adding a capacitance(C_(NEW)) 132 as shown in FIG. 8, the plate capacitance 116 is increasedwhile the motional capacitance 128 remains constant. By placing thecapacitance 132 in parallel with each targeted FBAR, the effectivecoupling coefficient of the FBAR filter is controllably reduced. Itshould be noted that by adding another resonator of the same frequencyin parallel with the FBAR, both of the capacitances 116 and 128 of FIG.7 are increased, so that the ratio will be unaffected. Clearly, acapacitor rather than a resonator provides the desired effects.

Preferably, the added capacitance 132 is fabricated using the samematerials and techniques as used in the fabrication of the FBAR. Theconcern with forming a capacitor using the top and bottom electrodelayers as plates and the piezoelectric layer as a dielectric is that aresonator will be formed, rather than a capacitor. Thus, the new deviceshould be mass loaded such that it does not resonate at one of thefrequencies of interest. Referring to FIG. 9, one technique for massloading the new device is to fabricate the capacitor stack 134 directlyon the surface of the substrate 136. This pulls the resonant frequencyoff of the center. That is, although the layers of the capacitor stack134 have the same thicknesses as the layers of the affected FBAR 138,the frequencies will be different because the capacitor stack is formeddirectly on the surface of the substrate 136, while the FBAR issuspended over the substrate by the formation of a well 140. In order toproperly distinguish the capacitor stack 134 from the FBAR stack 138,the connections that provide the electrical parallel arrangement are notshown.

The technique of forming the capacitor stack 134 directly on the surfaceof the substrate 136 allows the tailoring of the coupling coefficient ofthe affected FBAR filter, as described with reference to FIG. 8. Theconcern with this approach is that the capacitor stack may act as atransducer which broadcasts energy into the substrate 136. Since thesubstrate is relatively thick, a multitude of frequencies can betransmitted. Thus, the new device may be undesirably loosy.

Referring now to FIG. 10, another approach to forming the capacitance132 of FIG. 8 is to apply a gold layer 142 to the top of the capacitorstack that is formed over a well 144. The components of FIG. 10 that areidentical to those of FIG. 9 are provided with identical referencenumerals. Thus, the only differences between the approaches of FIGS. 9and 10 are the added gold layer 142 and the added well 144. Preferably,the gold layer is the same layer that is conventionally used to form thecontact pads for the array of FBARs. The addition of the gold layerreduces the resonant frequency of the stack 134, since it increases theweighted thickness of the top electrode. An advantage to the use of thegold from the pad layer metal level is that the frequency can be “tuned”so that it is not only off of the frequency of interest, but that itforms a parasitic resonance at frequencies where the duplexer does notperform well. As one example, the capacitor may resonate at 1510 MHz,which is a frequency at which existing duplexers do not perform well inthe rejection of energy. Tuning the capacitor stack 134 to 1510 MHzallows a designer to incorporate specific shunt resonators and seriesresonators that reduce leakage of the 1510 MHz signal. This is achievedwithout adding process steps to the FBAR fabrication. The tuning of thecapacitor stack can be achieved merely by properly selecting thethicknesses of the gold and other layers in the stack.

According to an example, the present invention includes a method forforming a piezoelectric layer to fabricate an acoustic resonator device.More specifically, the present method includes forming a single crystalmaterial to be used to fabricate the acoustic resonator device. Bymodifying the strain state of the III-Nitride (III-N) crystal lattice,the present method can change the piezoelectric properties of the singlecrystal material to adjust the acoustic properties of subsequent devicesfabricated from this material. In a specific example, the method forforming the strained single crystal material can include modification ofgrowth conditions of individual layers by employing one or a combinationof the following parameters; gas phase reactant ratios, growth pressure,growth temperature, and introduction of impurities.

In an example, the single crystal material is grown epitaxially upon asubstrate. Methods for growing the single crystal material can includemetal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy(MBE), hydride vapor phase epitaxy (HVPE), atomic layer deposition(ALD), or the like. Various process conditions can be selectively variedto change the piezoelectric properties of the single crystal material.These process conditions can include temperature, pressure, layerthickness, gas phase ratios, and the like. For example, the temperatureconditions for films containing aluminum (Al) and gallium (Ga) and theiralloys can range from about 800 to about 1500 degrees Celsius. Thetemperature conditions for films containing Al, Ga, and indium (In) andtheir alloys can range from about 600 to about 1000 degrees Celsius. Inanother example, the pressure conditions for films containing Al, Ga,and In and their alloys can range from about 1E-4 Torr to about 900Torr.

FIG. 11 is a flow diagram illustrating a method for manufacturing anacoustic resonator device according to an example of the presentinvention. The following steps are merely examples and should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.For example, various steps outlined below may be added, removed,modified, rearranged, repeated, and/or overlapped, as contemplatedwithin the scope of the invention. A typical growth process 1100 can beoutlined as follows:

-   -   1101. Provide a substrate having the required material        properties and crystallographic orientation. Various substrates        can be used in the present method for fabricating an acoustic        resonator device such as Silicon, Sapphire, Silicon Carbide,        Gallium Nitride (GaN) or Aluminum Nitride (AlN) bulk substrates.        The present method can also use GaN templates, AlN templates,        and Al_(x)Ga_(1-x)N templates (where x varies between 0.0 and        1.0). These substrates and templates can have polar, non-polar,        or semi-polar crystallographic orientations. Those of ordinary        skill in the art will recognize other variations, modifications,        and alternatives;    -   1102. Place the selected substrate into a processing chamber        within a controlled environment;    -   1103. Heat the substrate to a first desired temperature. At a        reduced pressure between 5-800 mbar the substrates are heated to        a temperature in the range of 1100°-1400° C. in the presence of        purified hydrogen gas as a means to clean the exposed surface of        the substrate. The purified hydrogen flow shall be in the range        of 5-30 slpm (standard liter per minute) and the purity of the        gas should exceed 99.9995%;    -   1104. Cool the substrate to a second desired temperature. After        10-15 minutes at elevated temperature, the substrate surface        temperature should be reduced by 100-200° C.; the temperature        offset here is determined by the selection of substrate material        and the initial layer to be grown (Highlighted in FIGS. 18A-C);    -   1105. Introduce reactants to the processing chamber. After the        temperature has stabilized the Group III and Group V reactants        are introduced to the processing chamber and growth is        initiated.    -   1106. Upon completion of the nucleation layer the growth chamber        pressures, temperatures, and gas phase mixtures may be further        adjusted to grow the layer or plurality of layers of interest        for the acoustic resonator device.    -   1107. During the film growth process the strain-state of the        material may be modulated via the modification of growth        conditions or by the controlled introduction of impurities into        the film (as opposed to the modification of the electrical        properties of the film).    -   1108. At the conclusion of the growth process the Group III        reactants are turned off and the temperature resulting film or        films are controllably lowered to room. The rate of thermal        change is dependent upon the layer or plurality of layers grown        and in the preferred embodiment is balanced such that the        physical parameters of the substrate including films are        suitable for subsequent processing.

Referring to step 1105, the growth of the single crystal material can beinitiated on a substrate through one of several growth methods: directgrowth upon a nucleation layer, growth upon a super lattice nucleationlayer, and growth upon a graded transition nucleation layer. The growthof the single crystal material can be homoepitaxial, heteroepitaxial, orthe like. In the homoepitaxial method, there is a minimal latticemismatch between the substrate and the films such as the case for anative III-N single crystal substrate material. In the heteroepitaxialmethod, there is a variable lattice mismatch between substrate and filmbased on in-plane lattice parameters. As further described below, thecombinations of layers in the nucleation layer can be used to engineerstrain in the subsequently formed structure.

Referring to step 1106, various substrates can be used in the presentmethod for fabricating an acoustic resonator device. Silicon substratesof various crystallographic orientations may be used. Additionally, thepresent method can use sapphire substrates, silicon carbide substrates,gallium nitride (GaN) bulk substrates, or aluminum nitride (AlN) bulksubstrates. The present method can also use GaN templates, AlNtemplates, and Al_(x)Ga_(1-x)N templates (where x varies between 0.0 and1.0). These substrates and templates can have polar, non-polar, orsemi-polar crystallographic orientations. Those of ordinary skill in theart will recognize other variations, modifications, and alternatives.

In an example, the present method involves controlling materialcharacteristics of the nucleation and piezoelectric layer(s). In aspecific example, these layers can include single crystal materials thatare configured with defect densities of less than 1E+11 defects persquare centimeter. The single crystal materials can include alloysselected from at least one of the following: AlN, AlGaN, GaN, InN,InGaN, AlInN, AlInGaN, ScAlN, ScAlGaN, ScN, BAlN, BAlScN, and BN. Invarious examples, any single or combination of the aforementionedmaterials can be used for the nucleation layer(s) and/or thepiezoelectric layer(s) of the device structure.

According to an example, the present method involves strain engineeringvia growth parameter modification. More specifically, the methodinvolves changing the piezoelectric properties of the epitaxial films inthe piezoelectric layer via modification of the film growth conditions(these modifications can be measured and compared via the sound velocityof the piezoelectric films). These growth conditions can includenucleation conditions and piezoelectric layer conditions. The nucleationconditions can include temperature, thickness, growth rate, gas phaseratio (V/III), and the like. The piezo electric layer conditions caninclude transition conditions from the nucleation layer, growthtemperature, layer thickness, growth rate, gas phase ratio (V/III), postgrowth annealing, and the like. Further details of the present methodcan be found below.

FIG. 12 is a simplified graph illustrating the results of forming apiezoelectric layer for an acoustic resonator device according to anexample of the present invention. This graph highlights the ability ofto tailor the acoustic properties of the material for a given Aluminummole fraction. Referring to step 1107 above, such flexibility allows forthe resulting resonator properties to be tailored to the individualapplication. As shown, graph 1200 depicts a plot of acoustic velocity(m/s) over aluminum mole fraction (%). The marked region 1220 shows themodulation of acoustic velocity via strain engineering of the piezoelectric layer at an aluminum mole fraction of 0.4. Here, the data showsthat the change in acoustic velocity ranges from about 7,500 m/s toabout 9,500 m/s, which is about ±1,000 m/s around the initial acousticvelocity of 8,500 m/s. Thus, the modification of the growth parametersprovides a large tunable range for acoustic velocity of the acousticresonator device. This tunable range will be present for all aluminummole fractions from 0 to 1.0 and is a degree of freedom not present inother conventional embodiments of this technology

The present method also includes strain engineering by impurityintroduction, or doping, to impact the rate at which a sound wave willpropagate through the material. Referring to step 1107 above, impuritiescan be specifically introduced to enhance the rate at which a sound wavewill propagate through the material. In an example, the impurity speciescan include, but is not limited to, the following: silicon (Si),magnesium (Mg), carbon (C), oxygen (O), erbium (Er), rubidium (Rb),strontium (Sr), scandium (Sc), beryllium (Be), molybdenum (Mo),zirconium (Zr), Hafnium (Hf), and vanadium (Va). Silicon, magnesium,carbon, and oxygen are common impurities used in the growth process, theconcentrations of which can be varied for different piezoelectricproperties. In a specific example, the impurity concentration rangesfrom about 1E+10 to about 1E+21 per cubic centimeter. The impuritysource used to deliver the impurities to can be a source gas, which canbe delivered directly, after being derived from an organometallicsource, or through other like processes.

The present method also includes strain engineering by the introductionof alloying elements, to impact the rate at which a sound wave willpropagate through the material. Referring to step 1107 above, alloyingelements can be specifically introduced to enhance the rate at which asound wave will propagate through the material. In an example, thealloying elements can include, but are not limited to, the following:magnesium (Mg), erbium (Er), rubidium (Rb), strontium (Sr), scandium(Sc), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (Va),niobium (Nb), and tantalum (Ta). In a specific embodiment, the alloyingelement (ternary alloys) or elements (in the case of quaternary alloys)concentration ranges from about 0.01% to about 50%. Similar to theabove, the alloy source used to deliver the alloying elements can be asource gas, which can be delivered directly, after being derived from anorganometallic source, or through other like processes. Those ofordinary skill in the art will recognize other variations,modifications, and alternatives to these processes.

The methods for introducing impurities can be during film growth(in-situ) or post growth (ex-situ). During film growth, the methods forimpurity introduction can include bulk doping, delta doping, co-doping,and the like. For bulk doping, a flow process can be used to create auniform dopant incorporation. For delta doping, flow processes can beintentionally manipulated for localized areas of higher dopantincorporation. For co-doping, the any doping methods can be used tosimultaneously introduce more than one dopant species during the filmgrowth process. Following film growth, the methods for impurityintroduction can include ion implantation, chemical treatment, surfacemodification, diffusion, co-doping, or the like. Those of ordinary skillin the art will recognize other variations, modifications, andalternatives.

FIG. 13A is a simplified diagram illustrating a method for forming apiezoelectric layer for an acoustic resonator device according to anexample of the present invention. As shown in device 1301, thepiezoelectric layer 1331, or film, is directly grown on the nucleationlayer 1321, which is formed overlying a surface region of a substrate1310. The nucleation layer 1321 may be the same or different atomiccomposition as the piezoelectric layer 1331. Here, the piezoelectricfilm 1331 may be doped by one or more species during the growth(in-situ) or post-growth (ex-situ) as described previously.

FIG. 13B is a simplified diagram illustrating a method for forming apiezoelectric layer for an acoustic resonator device according to anexample of the present invention. As shown in device 1302, thepiezoelectric layer 1332, or film, is grown on a super latticenucleation layer 1322, which is comprised of layer with alternatingcomposition and thickness. This super lattice layer 1322 is formedoverlying a surface region of the substrate 1310. The strain of device1302 can be tailored by the number of periods, or alternating pairs, inthe super lattice layer 1322 or by changing the atomic composition ofthe constituent layers. Similarly, the piezoelectric film 1332 may bedoped by one or more species during the growth (in-situ) or post-growth(ex-situ) as described previously.

FIG. 13C is a simplified diagram illustrating a method for forming apiezoelectric layer for an acoustic resonator device according to anexample of the present invention. As shown in device 1303, thepiezoelectric layer 1333, or film, is grown on graded transition layers1323. These transition layers 1323, which are formed overlying a surfaceregion of the substrate 1310, can be used to tailor the strain of device1303. In an example, the alloy (binary or ternary) content can bedecreased as a function of growth in the growth direction. This functionmay be linear, step-wise, or continuous. Similarly, the piezoelectricfilm 1333 may be doped by one or more species during the growth(in-situ) or post-growth (ex-situ) as described previously.

In an example, the present invention provides a method for manufacturingan acoustic resonator device. As described previously, the method caninclude a piezoelectric film growth process such as a direct growth upona nucleation layer, growth upon a super lattice nucleation layer, or agrowth upon graded transition nucleation layers. Each process can usenucleation layers that include, but are not limited to, materials oralloys having at least one of the following: AlN, AlGaN, GaN, InN,InGaN, AlInN, AlInGaN, ScAlN, ScAlGaN, ScN, BAlN, BAlScN, and BN. Thoseof ordinary skill in the art will recognize other variations,modifications, and alternatives.

In an example, the present invention provides a method of fabricating anarray of acoustic resonators. The method includes selecting a firsttarget frequency response and a first target effective couplingcoefficient for operating of a first single crystal film bulk acousticresonator (FBAR) filter of the array of acoustic resonators; andselecting a second target frequency response and a second targeteffective coupling coefficient for operation of a second single crystalFBAR filter of the array of acoustic resonators, said first targeteffective coupling coefficient being degraded relative to said secondtarget effective coupling coefficient.

In a specific example, said step of selecting said first targetcoefficient includes selecting a value in the range of 2.5 percent to10.0 percent and said step of selecting said second target coefficientincludes selecting a value in the range of 2.5 percent to 10.0 percent;and wherein said step of selecting said first and second targetcoefficients includes selecting the values such that the value of thefirst target coefficient is less than the value of the second targetcoefficient.

The method also includes determining thicknesses and materials of singlecrystal piezoelectric and electrode layers for forming said first andsecond single crystal FBAR filters so as to achieve said first andsecond target frequency responses and said first and second targeteffective coupling coefficients, including selecting an increasedelectrode layer thickness for at least one electrode layer of said firstsingle crystal FBAR filter, said increased electrode layer thicknessbeing greater than electrode layer thicknesses selected for said secondsingle crystal FBAR filter such that said degraded first targeteffective coupling coefficient is achieved; and forming said first andsecond single crystal FBAR filters based on said thicknesses andmaterials. Forming said first and second single crystal FBAR filtersbased on said thicknesses and materials includes forming said singlecrystal piezoelectric layers as strained single crystal piezoelectriclayers of said thicknesses and materials by a high temperaturemechanical sputtering process configured by initial surface etchingparameters and piezoelectric layer deposition parameters to modulate astrain condition in each of the strained piezoelectric layers to improveone or more piezoelectric properties of said strained piezoelectriclayer.

In a specific example, said step of determining said thicknesses andmaterials includes determining a ratio of a thickness of a singlecrystal piezoelectric layer to a total thickness of electrode layers foreach of said first and second single crystal FBAR filters, said ratiofor said first single crystal FBAR filter being less than said ratio forsaid second single crystal FBAR filter. In a specific example, said stepof forming said first and second single crystal FBAR filters includesdepositing AIN or AlScN as said single crystal piezoelectric layer anddepositing Mo as said electrode layers, said electrode layers of saidfirst single crystal FBAR filter having a thickness that is in the rangeof 1 to 3 times the thickness of electrode layers of said second singlecrystal FBAR filter. Also, in a specific example, said selecting stepsand said step of forming said first and second single crystal FBARfilters includes providing a first target frequency response having acenter frequency that is within 500 MHz of a center frequency of saidsecond target frequency response. Further, in a specific example,forming said first and second single crystal FBAR filters includesforming said strained single crystal piezoelectric layers of saidthicknesses and materials overlying a nucleation layer characterized bythe initial surface etching parameters, the nucleation layer beingformed overlying a substrate selected from one of the following: asilicon substrate, a sapphire substrate, silicon carbide substrate, aGaN bulk substrate, a GaN template, an AlN bulk, an AlN template, anAl_(x)Ga_(1-x)N template, and an silicon on insulator (SOI) wafer, andthe like.

In a specific example, the initial surface etching parameters includetemperature, pressure, thickness, etch rate, and etchant species, andthe like. In a specific example, the piezoelectric layer depositionparameters include nucleation layer formation and transition conditions,growth temperature, growth pressure, power, time, target-substratedistance, layer thickness, growth rate, and gas phase ratio, and thelike.

In specific example, the high temperature mechanical sputtering processincludes physical vapor deposition (PVD) and the like. Also, thenucleation layer and the strained single crystal piezoelectric layerscan include materials or alloys having at least one of the following:AlGaN, GaN, InN, InGaN, AlInN, AlInGaN, ScAlN, ScAlGaN, ScN, BAlN,BAlScN, and BN. In an example, the method can further include the stepof forming a duplexer to include each of: said first single crystal FBARfilter as a transmit filter; and said second single crystal FBAR filteras a receive filter.

In a specific example, one or more of the piezoelectric layers can beconfigured as a layer characterized by an x-ray diffraction (XRD)rocking curve full width at half maximum (FWHM) ranging from 0 degreesto 2 degrees. The x-ray rocking curve FWHM parameter can depend on thecombination of materials used for the piezoelectric layer and thesubstrate, as well as the thickness of these materials. Further, an FWHMprofile is used to characterize material properties and surfaceintegrity features, and is an indicator of crystal quality/purity. Theacoustic resonator devices using single crystal materials exhibit alower FWHM compared to devices using polycrystalline material, i.e.,single crystal materials have a higher crystal quality or crystalpurity. In a specific example, one or more of the single crystalpiezoelectric layers are characterized by an x-ray diffraction withclear peak at a detector angle (2-Theta) associated with a singlecrystal film and characterized by a Full Width Half Maximum (FWHM) ofless than 1.0°. Those of ordinary skill in the art will recognize othervariations, modifications, and alternatives to examples discussedpreviously.

In an example, the present invention provides a duplexer a transmitsingle crystal FBAR array and a receive single crystal FBAR array. Thetransmit single crystal FBAR array includes transmit series singlecrystal FBARs connected in series and having transmit shunt singlecrystal FBAR. The receive single crystal FBAR array includes receiveseries single crystal FBARs connected in series and having receive shuntsingle crystal FBARs. Also, said transmit single crystal FBAR array hasan effective coupling coefficient that is less than an effectivecoupling coefficient of said receive single crystal FBAR array, saidless effective coupling coefficient being realized by providing at leastsome single crystal FBARs of said transmit single crystal FBAR arraywith thicker electrode layers and thinner piezoelectric layers than saidsingle crystal FBARs of said receive single crystal FBAR array, said atleast some single crystal FBARs of said transmit single crystal FBARarray each having a ratio of thickness of said single crystalpiezoelectric layer to total thickness of said electrode layers, withsaid ratio being less than ratios of piezoelectric layer thickness tototal electrode layer thickness for said receive series single crystalFBARs and said receive shunt single crystal FBARs.

In an example, each said transmit shunt single crystal FBARs include afirst strained piezoelectric layer configured by first initial surfaceetching parameters and first piezoelectric layer deposition parametersto modulate a first strain condition to improve one or morepiezoelectric properties of said first strained piezoelectric layer.Also, said receive shunt single crystal FBARs include a second strainedpiezoelectric layer configured by second initial surface etchingparameters and second piezoelectric layer deposition parameters tomodulate a second strain condition to improve one or more piezoelectricproperties of said second strained piezoelectric layer.

In a specific example, said transmit and receive single crystal FBARarrays have frequencies compatible with operation in the spectrumdefined from 1 to 10 GHz. In a specific example, one or more of thesingle crystal piezoelectric layers are characterized by an x-raydiffraction with clear peak at a detector angle (2-Theta) associatedwith a single crystal film and characterized by a FWHM of less than1.0°. In a specific example, said total thickness of said electrodelayers of said at least some single crystal FBARs is in the range of 1to 3 times said total electrode layer thickness for said receive seriessingle crystal FBARs and said receive shunt single crystal FBARs. Also,in a specific example, the first and second nucleation layers and thefirst and second strained single crystal piezoelectric layers eachinclude materials or alloys having at least one of the following: AlN,AlGaN, GaN, InN, InGaN, AlInN, AlInGaN, ScAlN, ScAlGaN, ScN, BAlN,BAlScN, and BN.

In an example, the first strained single crystal piezoelectric layer iscoupled overlying a first nucleation layer characterized by the firstinitial surface etching parameters, the first nucleation layer beingcoupled overlying a first substrate. The second strained single crystalpiezoelectric layer is coupled overlying a second nucleation layercharacterized by the second initial surface etching parameters, thesecond nucleation layer being coupled overlying a second substrate.Further, the first and second substrates are each selected from one ofthe following: a silicon substrate, a sapphire substrate, siliconcarbide substrate, a GaN bulk substrate, a GaN template, an AlN bulk, anAlN template, an Al_(x)Ga_(1-x)N template, and an silicon on insulator(SOI) wafer.

In a specific example, the first and second initial surface etchingparameters include temperature, pressure, thickness, etch rate, andetchant species, and the like. In a specific example, the first andsecond piezoelectric layer deposition parameters include nucleationlayer formation and transition conditions, growth temperatures, growthpressure, power, time, target-substrate distance, layer thickness,growth rate, and gas phase ratio. There can be other variations,modifications, and alternatives to the examples discussed previously.

One or more benefits are achieved over pre-existing techniques using theinvention. In particular, the present device can be manufactured in arelatively simple and cost effective manner while using conventionalmaterials and/or methods according to one of ordinary skill in the art.Using the present method, one can create a reliable single crystal basedacoustic resonator using multiple ways of three-dimensional stackingthrough a wafer level process. Such filters or resonators can beimplemented in an RF filter device, an RF filter system, or the like.Depending upon the embodiment, one or more of these benefits may beachieved. Of course, there can be other variations, modifications, andalternatives.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. As an example, the packaged device can include any combination ofelements described above, as well as outside of the presentspecification. Therefore, the above description and illustrations shouldnot be taken as limiting the scope of the present invention which isdefined by the appended claims.

What is claimed is:
 1. A method of fabricating an array of acoustic resonators comprising the steps of: selecting a first target frequency response and a first target effective coupling coefficient for operation of a first single crystal film bulk acoustic resonator (FBAR) filter of the array of acoustic resonators; selecting a second target frequency response and a second target effective coupling coefficient for operation of a second single crystal FBAR filter of the array of acoustic resonators, said first target effective coupling coefficient being degraded relative to said second target effective coupling coefficient; determining thicknesses and materials of single crystal piezoelectric and electrode layers for forming said first and second single crystal FBAR filters so as to achieve said first and second target frequency responses and said first and second target effective coupling coefficients, including selecting an increased electrode layer thickness for at least one electrode layer of said first single crystal FBAR filter, said increased electrode layer thickness being greater than electrode layer thicknesses selected for said second single crystal FBAR filter such that said degraded first target effective coupling coefficient is achieved; and forming said first and second single crystal FBAR filters based on said thicknesses and materials; wherein forming said first and second single crystal FBAR filters based on said thicknesses and materials includes forming said single crystal piezoelectric layers as strained single crystal piezoelectric layers of said thicknesses and materials by a high temperature mechanical sputtering process configured by initial surface etching parameters and piezoelectric layer deposition parameters to modulate a strain condition in each of the strained piezoelectric layers to improve one or more piezoelectric properties of said strained piezoelectric layer.
 2. The method of claim 1 wherein said step of determining said thicknesses and materials includes determining a ratio of a thickness of a single crystal piezoelectric layer to a total thickness of electrode layers for each of said first and second single crystal FBAR filters, said ratio for said first single crystal FBAR filter being less than said ratio for said second single crystal FBAR filter.
 3. The method of claim 1 wherein one or more of the single crystal piezoelectric layers are characterized by an x-ray diffraction with clear peak at a detector angle (2-Theta) associated with a single crystal film and characterized by a Full Width Half Maximum (FWHM) of less than 1.0°.
 4. The method of claim 1 wherein said step of selecting said first target coefficient includes selecting a value in the range of 2.5 percent to 10.0 percent and said step of selecting said second target coefficient includes selecting a value in the range of 2.5 percent to 10.0 percent; and wherein said step of selecting said first and second target coefficients includes selecting the values such that the value of the first target coefficient is less than the value of the second target coefficient.
 5. The method of claim 1 further comprising the step of forming a duplexer to include each of: said first single crystal FBAR filter as a transmit filter; and said second single crystal FBAR filter as a receive filter.
 6. The method of claim 1 wherein said step of forming said first and second single crystal FBAR filters includes depositing AIN or AlScN as said single crystal piezoelectric layer and depositing Mo as said electrode layers, said electrode layers of said first single crystal FBAR filter having a thickness that is in the range of 1 to 3 times the thickness of electrode layers of said second single crystal FBAR filter.
 7. The method of claim 1 wherein said selecting steps and said step of forming said first and second single crystal FBAR filters includes providing a first target frequency response having a center frequency that is within 500 MHz of a center frequency of said second target frequency response.
 8. The method of claim 1 wherein forming said first and second single crystal FBAR filters includes forming said strained single crystal piezoelectric layers of said thicknesses and materials overlying a nucleation layer characterized by the initial surface etching parameters, the nucleation layer being formed overlying a substrate selected from one of the following: a silicon substrate, a sapphire substrate, silicon carbide substrate, a GaN bulk substrate, a GaN template, an AlN bulk, an AlN template, an Al_(x)Ga_(1-x)N template, and an silicon on insulator (SOI) wafer.
 9. The method of claim 8 wherein the high temperature mechanical sputtering process includes physical vapor deposition (PVD); and wherein the nucleation layer and the strained single crystal piezoelectric layers include materials or alloys having at least one of the following: AlN, AlGaN, GaN, InN, InGaN, AlInN, AlInGaN, ScAlN, ScAlGaN, ScN, BAlN, BAlScN, and BN.
 10. The method of claim 1 wherein the initial surface etching parameters include temperature, pressure, thickness, etch rate, and etchant species.
 11. The method of claim 1 wherein the piezoelectric layer deposition parameters include nucleation layer formation and transition conditions, growth temperature, growth pressure, power, time, target-substrate distance, layer thickness, growth rate, and gas phase ratio.
 12. A duplexer comprising: a transmit single crystal film bulk acoustic resonator (FBAR) array having transmit series single crystal FBARs connected in series and having transmit shunt single crystal FBARS; and a receive single crystal FBAR array having receive series single crystal FBARs connected in series and having receive shunt single crystal FBARs; wherein said transmit single crystal FBAR array has an effective coupling coefficient that is less than an effective coupling coefficient of said receive single crystal FBAR array, said less effective coupling coefficient being realized by providing at least two single crystal FBARs of said transmit single crystal FBAR array with thicker electrode layers and thinner piezoelectric layers than said single crystal FBARs of said receive single crystal FBAR array, said at least two single crystal FBARs of said transmit single crystal FBAR array each having a ratio of thickness of said single crystal piezoelectric layer to total thickness of said electrode layers, with said ratio being less than ratios of piezoelectric layer thickness to total electrode layer thickness for said receive series single crystal FBARs and said receive shunt single crystal FBARs; wherein each said transmit shunt single crystal FBARs include a first strained piezoelectric layer configured by first initial surface etching parameters and first piezoelectric layer deposition parameters to modulate a first strain condition to improve one or more piezoelectric properties of said first strained piezoelectric layer; and wherein each said receive shunt single crystal FBARs include a second strained piezoelectric layer configured by second initial surface etching parameters and second piezoelectric layer deposition parameters to modulate a second strain condition to improve one or more piezoelectric properties of said second strained piezoelectric layer.
 13. The duplexer of claim 12 wherein said transmit and receive single crystal FBAR arrays have frequencies compatible with operation in the spectrum defined from 1 to 10 GHz.
 14. The duplexer of claim 12 wherein one or more of the single crystal piezoelectric layers are characterized by an x-ray diffraction with clear peak at a detector angle (2-Theta) associated with a single crystal film and characterized by a Full Width Half Maximum (FWHM) of less than 1.0°.
 15. The duplexer of claim 12 wherein said total thickness of said electrode layers of said at least two single crystal FBARs is in the range of 1 to 3 times said total electrode layer thickness for said receive series single crystal FBARs and said receive shunt single crystal FBARs.
 16. The duplexer of claim 12 wherein the first strained single crystal piezoelectric layer is coupled overlying a first nucleation layer characterized by the first initial surface etching parameters, the first nucleation layer being coupled overlying a first substrate; wherein the second strained single crystal piezoelectric layer is coupled overlying a second nucleation layer characterized by the second initial surface etching parameters, the second nucleation layer being coupled overlying a second substrate; wherein the first and second substrates are each selected from one of the following: a silicon substrate, a sapphire substrate, silicon carbide substrate, a GaN bulk substrate, a GaN template, an AlN bulk, an AlN template, an Al_(x)Ga_(1-x)N template, and an silicon on insulator (SOI) wafer.
 17. The duplexer of claim 16 wherein the first and second nucleation layers and the first and second strained single crystal piezoelectric layers each include materials or alloys having at least one of the following: AlN, AlGaN, GaN, InN, InGaN, AlInN, AlInGaN, ScAlN, ScAlGaN, ScN, BAlN, BAlScN, and BN.
 18. The duplexer of claim 12 wherein the first and second initial surface etching parameters include temperature, pressure, thickness, etch rate, and etchant species.
 19. The duplexer of claim 12 wherein the first and second piezoelectric layer deposition parameters include nucleation layer formation and transition conditions, growth temperatures, growth pressure, power, time, target-substrate distance, layer thickness, growth rate, and gas phase ratio.
 20. A duplexer comprising: a transmit single crystal film bulk acoustic resonator (FBAR) array having transmit series single crystal FBARs connected in series and having transmit shunt single crystal FBARs; and a receive single crystal FBAR array having receive series single crystal FBARs connected in series and having receive shunt single crystal FBARs; wherein said transmit single crystal FBAR array has an effective coupling coefficient that is less than an effective coupling coefficient of said receive single crystal FBAR array, at least some single crystal FBARs of said transmit single crystal FBAR array having thicker electrode layers and thinner single crystal piezoelectric layers than said single crystal FBARs of said receive single crystal FBAR array in order to attain said less effective coupling coefficient that is exhibited by said transmit single crystal FBAR array; wherein each said transmit shunt single crystal FBARs include a first strained piezoelectric layer configured by first initial surface etching parameters and first piezoelectric layer deposition parameters to modulate a first strain condition to improve one or more piezoelectric properties of said first strained piezoelectric layer; and wherein each said receive shunt single crystal FBARs include a second strained piezoelectric layer configured by second initial surface etching parameters and second piezoelectric layer deposition parameters to modulate a second strain condition to improve one or more piezoelectric properties of said second strained piezoelectric layer.
 21. The duplexer of claim 20 wherein the first strained single crystal piezoelectric layer is coupled overlying a first nucleation layer characterized by the first nucleation growth parameters, the first nucleation layer being coupled overlying a first substrate; wherein the second strained single crystal piezoelectric layer is coupled overlying a second nucleation layer characterized by the second nucleation growth parameters, the second nucleation layer being coupled overlying a second substrate; wherein the first and second substrates are each selected from one of the following: a silicon substrate, a sapphire substrate, silicon carbide substrate, a GaN bulk substrate, a GaN template, an AlN bulk, an AlN template, and an Al_(x)Ga_(1-x)N template.
 22. The duplexer of claim 20 wherein the first and second initial surface etching parameters include temperature, pressure, thickness, etch rate, and etchant species; and wherein the first and second piezoelectric layer deposition parameters include nucleation layer formation and transition conditions, growth temperatures, growth pressure, power, time, target-substrate distance, layer thickness, growth rate, and gas phase ratio.
 23. The duplexer of claim 20 wherein one or more of the single crystal piezoelectric layers are characterized by an x-ray diffraction with clear peak at a detector angle (2-Theta) associated with a single crystal film and characterized by a Full Width Half Maximum (FWHM) of less than 1.0°. 